8TR UCTTJRE OF THE BLOOD- VESSELS. 217 



red as "venous blood:" and it must be borne in mind that 

 the terms have this peculiar technical meaning, and that 

 the pulmonary veins contain arterial blood and the pulmo- 

 nary arteries, venom blood; the change from arterial to 

 venous taking place in the systemic capillaries, and from 

 venous to arterial in the pulmonary capillaries. The 

 chambers of the heart and the great vessels containing ar- 

 terial blood are shaded red in Figs. 79 and 80. 



The Structure of the Arteries. A large artery can by 

 careful dissection be separated into three coats; an internal, 

 middle and outer. The internal coat tears readily across 

 the long axis of the artery and consists of an inner lining 

 of flattened nucleated cells, and of a variable number of 

 layers composed of membranes or networks of elastic tissue, 

 outside this. The middle coat is made up of alternating 

 layers of elastic fibres and plain muscular tissue; the for- 

 mer running for the most part longitudinally and the latter 

 across the long axis of the vessel. The outer coat is the 

 toughest and strongest of all and is mainly made up of 

 white fibrous connective tissue but contains a considerable 

 amount of elastic tissue also. It gradually shades off into 

 a loose areolar tissue which forms the sheath of the artery 

 or the tunica adventitia, and packs it between surrounding 

 parts. The smaller arteries have all the elastic elements 

 less developed. The internal coat is consequently thinner, 

 and the middle coat is made up mainly of involuntary mus- 

 cular fibres. As a result the large arteries are highly elas- 

 tic, the aorta being physically much like a piece of indian- 

 rubber tubing, while the smaller arteries are highly con- 

 tractile, in the physiological sense of the word. 



Structure of the Capillaries. In the smaller arteries 

 the outer and middle coats gradually disappear, and the 

 elastic layers of the inner coat also go. Finally, in the 

 capillaries the lining epithelium alone is left, with a 

 more or less developed layer of connective-tissue corpuscles 

 around it, representing the remnant of the tunica adven- 

 titia. These vessels are thus extremely well adapted to al- 

 low of filtration or diffusion taking place through their 

 thin walls. 



